diff --git a/test/wcc/742_parse_error.c b/test/wcc/742_parse_error.c
index 2427ee38..50694076 100644
--- a/test/wcc/742_parse_error.c
+++ b/test/wcc/742_parse_error.c
@@ -12,6 +12,7 @@
  */
 #include <stdio.h>
 #include <stdlib.h>
+#include <string.h>
 #include <unistd.h>
 #include <sys/wait.h>
 
@@ -34,17 +35,41 @@ write_file(const char *path, const char *content)
 	return 0;
 }
 
+static int
+filehas(const char *path, const char *needle)
+{
+	char buf[8192];
+	FILE *f = fopen(path, "rb");
+	if (!f) return 0;
+	size_t n = fread(buf, 1, sizeof buf - 1, f);
+	fclose(f);
+	buf[n] = '\0';
+	return strstr(buf, needle) != NULL;
+}
+
+/* check_fail — the tool must exit non-zero AND emit a parse diagnostic.
+ * Both stages report "expected identifier" first on this fixture; a crash
+ * exits non-zero with no diagnostic, so the substring distinguishes a
+ * clean reject from a segfault (#20). */
 static int
 check_fail(const char *tool, const char *src)
 {
-	char cmd[4096];
-	snprintf(cmd, sizeof cmd, "%s %s >/dev/null 2>/dev/null", tool, src);
+	char cmd[4096], errf[64];
+	snprintf(errf, sizeof errf, "/tmp/parse_err_e_%d.txt", getpid());
+	snprintf(cmd, sizeof cmd, "%s %s >/dev/null 2>%s", tool, src, errf);
 	int rc = runwait(cmd);
+	int hasmsg = filehas(errf, "expected identifier");
+	unlink(errf);
 	if (rc == 0) {
 		fprintf(stderr, "742 FAIL: %s exited 0 on parse-error fixture\n",
 		    tool);
 		return -1;
 	}
+	if (!hasmsg) {
+		fprintf(stderr, "742 FAIL: %s nonzero exit but missing parse "
+		    "diagnostic (a crash, not a clean reject)\n", tool);
+		return -1;
+	}
 	return 0;
 }
 
diff --git a/test/wcc/814_def_arr_infer_len.c b/test/wcc/814_def_arr_infer_len.c
index 24b7eef5..e27aab41 100644
--- a/test/wcc/814_def_arr_infer_len.c
+++ b/test/wcc/814_def_arr_infer_len.c
@@ -158,6 +158,12 @@ static const char *neg[] = {
 	"export fn main() i32 = { return 0; };\n",
 };
 
+/* Per-neg expected diagnostic body (parallel to neg[]); shared by both
+ * stages (cstage prepends file:line:col, wwstage does not — #20). */
+static const char *neg_diag[] = {
+	"[_]T needs an array-literal initialiser",
+};
+
 static int
 run_driver(const char *driver, const struct row *r, int i)
 {
@@ -192,15 +198,30 @@ run_driver(const char *driver, const struct row *r, int i)
 	return got;
 }
 
-/* build_should_fail — a `def [_]T` that can't infer must error on
- * `driver`; returns 0 when the build correctly FAILS, non-zero when it
- * wrongly succeeded. */
 static int
-build_should_fail(const char *driver, const char *src, int i)
+filehas(const char *path, const char *needle)
 {
-	char s[64], tmpdir[64], cmd[1024];
+	char buf[8192];
+	FILE *f = fopen(path, "rb");
+	if (!f) return 0;
+	size_t n = fread(buf, 1, sizeof buf - 1, f);
+	fclose(f);
+	buf[n] = '\0';
+	return strstr(buf, needle) != NULL;
+}
+
+/* build_should_fail — a `def [_]T` that can't infer must error on
+ * `driver`; returns 0 when the build FAILS *and* emits `diag`. A crash
+ * (segfault) wraps as a nonzero "w6c failed" with no diagnostic, so the
+ * substring check distinguishes it from a clean reject (#20). */
+static int
+build_should_fail(const char *driver, const char *src, const char *diag,
+    int i)
+{
+	char s[64], tmpdir[64], cmd[1024], errf[64];
 	snprintf(s, sizeof s, "/tmp/dailn_%d_%d.ww", getpid(), i);
 	snprintf(tmpdir, sizeof tmpdir, "/tmp/dailn_%d_d_%d", getpid(), i);
+	snprintf(errf, sizeof errf, "/tmp/dailn_%d_e_%d.txt", getpid(), i);
 
 	FILE *f = fopen(s, "wb");
 	if (!f) return -1;
@@ -208,10 +229,12 @@ build_should_fail(const char *driver, const char *src, int i)
 	fclose(f);
 
 	mkdir(tmpdir, 0755);
-	snprintf(cmd, sizeof cmd, "cd %s && %s build %s 2>/dev/null",
-	    tmpdir, driver, s);
+	snprintf(cmd, sizeof cmd, "cd %s && %s build %s >/dev/null 2>%s",
+	    tmpdir, driver, s, errf);
 	int rc = runwait(cmd);
+	int hasmsg = filehas(errf, diag);
 	unlink(s);
+	unlink(errf);
 	/* clean any emitted binary */
 	const char *base = strrchr(s, '/');
 	base = base ? base + 1 : s;
@@ -221,7 +244,8 @@ build_should_fail(const char *driver, const char *src, int i)
 	if (dot && strcmp(dot, ".ww") == 0) *dot = '\0';
 	unlink(outbin);
 	rmdir(tmpdir);
-	return rc == 0 ? -1 : 0;	/* build must NOT succeed */
+	/* build must NOT succeed AND must emit its diagnostic. */
+	return (rc != 0 && hasmsg) ? 0 : -1;
 }
 
 /* asm_byte_identical — w6c vs w6c_ww .s for the same source must match.
@@ -326,7 +350,7 @@ main(void)
 		for (int i = 0; i < nn; i++) {
 			total++;
 			if (build_should_fail(drivers[d].path, neg[i],
-			    100 + i) != 0) {
+			    neg_diag[i], 100 + i) != 0) {
 				fprintf(stderr,
 				    "def_arr_infer_len[%s][neg%d]: built ok, "
 				    "expected a loud error\n",
diff --git a/test/wcc/817_arr_cap_reject.c b/test/wcc/817_arr_cap_reject.c
index 2dd546bb..fe3de25c 100644
--- a/test/wcc/817_arr_cap_reject.c
+++ b/test/wcc/817_arr_cap_reject.c
@@ -121,6 +121,15 @@ static const char *neg[] = {
 	"};\n",
 };
 
+/* Per-neg expected diagnostic body (parallel to neg[]); all four rows hit
+ * the same "no field 'cap' on a fixed-size array" path on both stages. */
+static const char *neg_diag[] = {
+	"no field 'cap' on a fixed-size array",
+	"no field 'cap' on a fixed-size array",
+	"no field 'cap' on a fixed-size array",
+	"no field 'cap' on a fixed-size array",
+};
+
 static int
 run_driver(const char *driver, const struct row *r, int i)
 {
@@ -155,14 +164,30 @@ run_driver(const char *driver, const struct row *r, int i)
 	return got;
 }
 
-/* build_should_fail — `.cap` on an array must error on `driver`; returns 0
- * when the build correctly FAILS, non-zero when it wrongly succeeded. */
 static int
-build_should_fail(const char *driver, const char *src, int i)
+filehas(const char *path, const char *needle)
 {
-	char s[64], tmpdir[64], cmd[1024];
+	char buf[8192];
+	FILE *f = fopen(path, "rb");
+	if (!f) return 0;
+	size_t n = fread(buf, 1, sizeof buf - 1, f);
+	fclose(f);
+	buf[n] = '\0';
+	return strstr(buf, needle) != NULL;
+}
+
+/* build_should_fail — `.cap` on an array must error on `driver`; returns 0
+ * when the build FAILS *and* emits `diag`. A crash (segfault) wraps as a
+ * nonzero "w6c failed" with no diagnostic, so the substring distinguishes
+ * it from a clean reject (#20). */
+static int
+build_should_fail(const char *driver, const char *src, const char *diag,
+    int i)
+{
+	char s[64], tmpdir[64], cmd[1024], errf[64];
 	snprintf(s, sizeof s, "/tmp/acrn_%d_%d.ww", getpid(), i);
 	snprintf(tmpdir, sizeof tmpdir, "/tmp/acrn_%d_d_%d", getpid(), i);
+	snprintf(errf, sizeof errf, "/tmp/acrn_%d_e_%d.txt", getpid(), i);
 
 	FILE *f = fopen(s, "wb");
 	if (!f) return -1;
@@ -170,10 +195,12 @@ build_should_fail(const char *driver, const char *src, int i)
 	fclose(f);
 
 	mkdir(tmpdir, 0755);
-	snprintf(cmd, sizeof cmd, "cd %s && %s build %s 2>/dev/null",
-	    tmpdir, driver, s);
+	snprintf(cmd, sizeof cmd, "cd %s && %s build %s >/dev/null 2>%s",
+	    tmpdir, driver, s, errf);
 	int rc = runwait(cmd);
+	int hasmsg = filehas(errf, diag);
 	unlink(s);
+	unlink(errf);
 	/* clean any emitted binary */
 	const char *base = strrchr(s, '/');
 	base = base ? base + 1 : s;
@@ -183,7 +210,8 @@ build_should_fail(const char *driver, const char *src, int i)
 	if (dot && strcmp(dot, ".ww") == 0) *dot = '\0';
 	unlink(outbin);
 	rmdir(tmpdir);
-	return rc == 0 ? -1 : 0;	/* build must NOT succeed */
+	/* build must NOT succeed AND must emit its diagnostic. */
+	return (rc != 0 && hasmsg) ? 0 : -1;
 }
 
 /* asm_byte_identical — w6c vs w6c_ww .s for the same source must match. */
@@ -286,7 +314,7 @@ main(void)
 		for (int i = 0; i < nn; i++) {
 			total++;
 			if (build_should_fail(drivers[d].path, neg[i],
-			    100 + i) != 0) {
+			    neg_diag[i], 100 + i) != 0) {
 				fprintf(stderr,
 				    "arr_cap_reject[%s][neg%d]: built ok, "
 				    "expected a loud error\n",
diff --git a/test/wcc/821_def_str_index_reject.c b/test/wcc/821_def_str_index_reject.c
index aa576ca9..5b12ab1d 100644
--- a/test/wcc/821_def_str_index_reject.c
+++ b/test/wcc/821_def_str_index_reject.c
@@ -141,6 +141,13 @@ static const char *neg[] = {
 	"};\n",
 };
 
+/* Per-neg expected diagnostic body (parallel to neg[]); both rows hit the
+ * same "cannot index a def-constant str" path on both stages. */
+static const char *neg_diag[] = {
+	"cannot index a def-constant str",
+	"cannot index a def-constant str",
+};
+
 static int
 run_driver(const char *driver, const struct row *r, int i)
 {
@@ -175,15 +182,30 @@ run_driver(const char *driver, const struct row *r, int i)
 	return got;
 }
 
-/* build_should_fail — indexing a def-global scalar str must error on
- * `driver`; returns 0 when the build correctly FAILS, non-zero when it
- * wrongly succeeded. */
 static int
-build_should_fail(const char *driver, const char *src, int i)
+filehas(const char *path, const char *needle)
 {
-	char s[64], tmpdir[64], cmd[1024];
+	char buf[8192];
+	FILE *f = fopen(path, "rb");
+	if (!f) return 0;
+	size_t n = fread(buf, 1, sizeof buf - 1, f);
+	fclose(f);
+	buf[n] = '\0';
+	return strstr(buf, needle) != NULL;
+}
+
+/* build_should_fail — indexing a def-global scalar str must error on
+ * `driver`; returns 0 when the build FAILS *and* emits `diag`. A crash
+ * (segfault) wraps as a nonzero "w6c failed" with no diagnostic, so the
+ * substring distinguishes it from a clean reject (#20). */
+static int
+build_should_fail(const char *driver, const char *src, const char *diag,
+    int i)
+{
+	char s[64], tmpdir[64], cmd[1024], errf[64];
 	snprintf(s, sizeof s, "/tmp/dsin_%d_%d.ww", getpid(), i);
 	snprintf(tmpdir, sizeof tmpdir, "/tmp/dsin_%d_d_%d", getpid(), i);
+	snprintf(errf, sizeof errf, "/tmp/dsin_%d_e_%d.txt", getpid(), i);
 
 	FILE *f = fopen(s, "wb");
 	if (!f) return -1;
@@ -191,10 +213,12 @@ build_should_fail(const char *driver, const char *src, int i)
 	fclose(f);
 
 	mkdir(tmpdir, 0755);
-	snprintf(cmd, sizeof cmd, "cd %s && %s build %s 2>/dev/null",
-	    tmpdir, driver, s);
+	snprintf(cmd, sizeof cmd, "cd %s && %s build %s >/dev/null 2>%s",
+	    tmpdir, driver, s, errf);
 	int rc = runwait(cmd);
+	int hasmsg = filehas(errf, diag);
 	unlink(s);
+	unlink(errf);
 	/* clean any emitted binary */
 	const char *base = strrchr(s, '/');
 	base = base ? base + 1 : s;
@@ -204,7 +228,8 @@ build_should_fail(const char *driver, const char *src, int i)
 	if (dot && strcmp(dot, ".ww") == 0) *dot = '\0';
 	unlink(outbin);
 	rmdir(tmpdir);
-	return rc == 0 ? -1 : 0;	/* build must NOT succeed */
+	/* build must NOT succeed AND must emit its diagnostic. */
+	return (rc != 0 && hasmsg) ? 0 : -1;
 }
 
 /* asm_byte_identical — w6c vs w6c_ww .s for the same source must match. */
@@ -307,7 +332,7 @@ main(void)
 		for (int i = 0; i < nn; i++) {
 			total++;
 			if (build_should_fail(drivers[d].path, neg[i],
-			    100 + i) != 0) {
+			    neg_diag[i], 100 + i) != 0) {
 				fprintf(stderr,
 				    "def_str_index_reject[%s][neg%d]: built ok, "
 				    "expected a loud error\n",
diff --git a/test/wcc/842_dup_main_reject.c b/test/wcc/842_dup_main_reject.c
index 9ca3b121..ec2dda27 100644
--- a/test/wcc/842_dup_main_reject.c
+++ b/test/wcc/842_dup_main_reject.c
@@ -62,17 +62,32 @@ runwait(const char *cmd)
 	return -1;
 }
 
+static int
+filehas(const char *path, const char *needle)
+{
+	char buf[8192];
+	FILE *f = fopen(path, "rb");
+	if (!f) return 0;
+	size_t n = fread(buf, 1, sizeof buf - 1, f);
+	fclose(f);
+	buf[n] = '\0';
+	return strstr(buf, needle) != NULL;
+}
+
 /* build_should_fail — a dup-`main` program must error on `driver`
- * (loud reject); returns 0 when the build correctly FAILS, non-zero
- * when it wrongly succeeded. Intermediates land in tmpdir, never the
- * source tree (rule 14). */
+ * (loud reject); returns 0 when the build FAILS *and* emits `diag`.
+ * This test drives the full `ww` driver, so a crash inside w6c also
+ * surfaces as the driver's "w6c failed" exit 1 — the diagnostic-substring
+ * check is the ONLY way to tell a crash from a clean reject here (#20).
+ * Intermediates land in tmpdir, never the source tree (rule 14). */
 static int
 build_should_fail(const char *driver, const char *label, const char *src,
-    int i)
+    const char *diag, int i)
 {
-	char s[64], tmpdir[64], cmd[1024];
+	char s[64], tmpdir[64], cmd[1024], errf[64];
 	snprintf(s, sizeof s, "/tmp/dupmain_%d_%d.ww", getpid(), i);
 	snprintf(tmpdir, sizeof tmpdir, "/tmp/dupmain_%d_d_%d", getpid(), i);
+	snprintf(errf, sizeof errf, "/tmp/dupmain_%d_e_%d.txt", getpid(), i);
 
 	FILE *f = fopen(s, "wb");
 	if (!f) return -1;
@@ -80,9 +95,10 @@ build_should_fail(const char *driver, const char *label, const char *src,
 	fclose(f);
 
 	mkdir(tmpdir, 0755);
-	snprintf(cmd, sizeof cmd, "cd %s && %s build %s 2>/dev/null",
-	    tmpdir, driver, s);
+	snprintf(cmd, sizeof cmd, "cd %s && %s build %s >/dev/null 2>%s",
+	    tmpdir, driver, s, errf);
 	int rc = runwait(cmd);
+	int hasmsg = filehas(errf, diag);
 
 	const char *base = strrchr(s, '/');
 	base = base ? base + 1 : s;
@@ -91,13 +107,22 @@ build_should_fail(const char *driver, const char *label, const char *src,
 	char *dot = strrchr(outbin, '.');
 	if (dot && strcmp(dot, ".ww") == 0) *dot = '\0';
 	unlink(s);
+	unlink(errf);
 	unlink(outbin);
 	rmdir(tmpdir);
-	if (rc == 0)
+	if (rc == 0) {
 		fprintf(stderr,
 		    "dup_main[%s][%s]: built ok, expected a loud reject\n",
 		    driver, label);
-	return rc == 0 ? -1 : 0;	/* build must NOT succeed */
+		return -1;
+	}
+	if (!hasmsg) {
+		fprintf(stderr, "dup_main[%s][%s]: nonzero exit but missing "
+		    "diagnostic \"%s\" (a crash, not a clean reject)\n",
+		    driver, label, diag);
+		return -1;
+	}
+	return 0;
 }
 
 /* run_build — build a program that MUST compile, run it, return its
@@ -134,7 +159,7 @@ run_build(const char *driver, const char *label, const char *src, int i)
 	return got;
 }
 
-struct rejrow { const char *label; const char *src; };
+struct rejrow { const char *label; const char *src; const char *diag; };
 
 /* Each program declares a second top-level `main` (in package foo)
  * alongside the entry `fn main` (in package main), bound by a REAL
@@ -150,7 +175,8 @@ static const struct rejrow reject_rows[] = {
 	  "export fn anchor() i32 = { return main; };\n"
 	  "package main;\n"
 	  "import foo;\n"
-	  "fn main() i32 = { return foo.anchor(); };\n" },
+	  "fn main() i32 = { return foo.anchor(); };\n",
+	  "duplicate entry main" },
 
 	{ "fn_main",
 	  "package foo;\n"
@@ -158,7 +184,8 @@ static const struct rejrow reject_rows[] = {
 	  "export fn anchor() i32 = { return 2; };\n"
 	  "package main;\n"
 	  "import foo;\n"
-	  "fn main() i32 = { return foo.anchor(); };\n" },
+	  "fn main() i32 = { return foo.anchor(); };\n",
+	  "duplicate entry main" },
 
 	{ "def_main",
 	  "package foo;\n"
@@ -166,7 +193,8 @@ static const struct rejrow reject_rows[] = {
 	  "export fn anchor() i32 = { return main; };\n"
 	  "package main;\n"
 	  "import foo;\n"
-	  "fn main() i32 = { return foo.anchor(); };\n" },
+	  "fn main() i32 = { return foo.anchor(); };\n",
+	  "duplicate entry main" },
 
 	{ "type_main",
 	  "package foo;\n"
@@ -174,7 +202,8 @@ static const struct rejrow reject_rows[] = {
 	  "export fn anchor() i32 = { return 3; };\n"
 	  "package main;\n"
 	  "import foo;\n"
-	  "fn main() i32 = { return foo.anchor(); };\n" },
+	  "fn main() i32 = { return foo.anchor(); };\n",
+	  "duplicate entry main" },
 };
 
 struct okrow { const char *label; const char *src; int want; };
@@ -225,7 +254,7 @@ main(void)
 			total++;
 			if (build_should_fail(drivers[d].path,
 			    reject_rows[i].label, reject_rows[i].src,
-			    d * 100 + i) != 0)
+			    reject_rows[i].diag, d * 100 + i) != 0)
 				fail++;
 		}
 		for (int i = 0; i < nok; i++) {
diff --git a/test/wcc/845_tuple_trailing_comma_reject.c b/test/wcc/845_tuple_trailing_comma_reject.c
index d5d0e3d9..632be4bf 100644
--- a/test/wcc/845_tuple_trailing_comma_reject.c
+++ b/test/wcc/845_tuple_trailing_comma_reject.c
@@ -50,30 +50,59 @@ outbin(char *dst, size_t n, const char *tmpdir, const char *src)
 	if (dot && strcmp(dot, ".ww") == 0) *dot = '\0';
 }
 
-/* build_should_fail — returns 0 when the build correctly FAILS. */
+static int
+filehas(const char *path, const char *needle)
+{
+	char buf[8192];
+	FILE *f = fopen(path, "rb");
+	if (!f) return 0;
+	size_t n = fread(buf, 1, sizeof buf - 1, f);
+	fclose(f);
+	buf[n] = '\0';
+	return strstr(buf, needle) != NULL;
+}
+
+/* build_should_fail — returns 0 when the build FAILS *and* the stage's
+ * own parse diagnostic appears; a crash wraps as a nonzero "w6c failed"
+ * with no diagnostic, so the substring distinguishes it (#20). The two
+ * stages' parse messages diverge with no common core (cstage "unexpected
+ * token in expression"/"expected ), got ;" vs wwstage "parse: expected
+ * expression"/"parse: expected ')' in tuple"), so `diag` is passed
+ * per-stage by the caller (a real fidelity gap, filed under #20). */
 static int
 build_should_fail(const char *driver, const char *label, const char *src,
-    int i)
+    const char *diag, int i)
 {
-	char s[64], tmpdir[64], cmd[1024], bin[128];
+	char s[64], tmpdir[64], cmd[1024], bin[128], errf[64];
 	snprintf(s, sizeof s, "/tmp/tuptc_%d_%d.ww", getpid(), i);
 	snprintf(tmpdir, sizeof tmpdir, "/tmp/tuptc_%d_d_%d", getpid(), i);
+	snprintf(errf, sizeof errf, "/tmp/tuptc_%d_e_%d.txt", getpid(), i);
 	FILE *f = fopen(s, "wb");
 	if (!f) return -1;
 	fputs(src, f);
 	fclose(f);
 	mkdir(tmpdir, 0755);
-	snprintf(cmd, sizeof cmd, "cd %s && %s build %s 2>/dev/null",
-	    tmpdir, driver, s);
+	snprintf(cmd, sizeof cmd, "cd %s && %s build %s >/dev/null 2>%s",
+	    tmpdir, driver, s, errf);
 	int rc = runwait(cmd);
+	int hasmsg = filehas(errf, diag);
 	outbin(bin, sizeof bin, tmpdir, s);
 	unlink(s);
 	unlink(bin);
+	unlink(errf);
 	rmdir(tmpdir);
-	if (rc == 0)
+	if (rc == 0) {
 		fprintf(stderr, "tuptc[%s][%s]: built ok, expected a reject\n",
 		    driver, label);
-	return rc == 0 ? -1 : 0;
+		return -1;
+	}
+	if (!hasmsg) {
+		fprintf(stderr, "tuptc[%s][%s]: nonzero exit but missing "
+		    "diagnostic \"%s\" (a crash, not a clean reject)\n",
+		    driver, label, diag);
+		return -1;
+	}
+	return 0;
 }
 
 /* run_build — build a program that MUST compile, run it, return exit. */
@@ -102,12 +131,18 @@ run_build(const char *driver, const char *label, const char *src, int i)
 	return got;
 }
 
-struct rejrow { const char *label; const char *src; };
+struct rejrow {
+	const char *label;
+	const char *src;
+	const char *cdiag;	/* cstage diagnostic body substring */
+	const char *wdiag;	/* wwstage diagnostic body substring (#20 gap) */
+};
 
 static const struct rejrow reject_rows[] = {
 	{ "one_comma",
 	  "package main;\n"
-	  "fn main() i32 = { let t = (5,); return 0; };\n" },
+	  "fn main() i32 = { let t = (5,); return 0; };\n",
+	  "unexpected token in expression", "expected expression" },
 };
 
 struct okrow { const char *label; const char *src; int want; };
@@ -161,7 +196,8 @@ main(void)
 			total++;
 			if (build_should_fail(drivers[d].path,
 			    reject_rows[i].label, reject_rows[i].src,
-			    d * 100 + i) != 0)
+			    drivers[d].gated ? reject_rows[i].wdiag
+			    : reject_rows[i].cdiag, d * 100 + i) != 0)
 				fail++;
 		}
 		for (int i = 0; i < nok; i++) {
diff --git a/test/wcc/846_voidless_return_reject.c b/test/wcc/846_voidless_return_reject.c
index f85b05c7..25cb6657 100644
--- a/test/wcc/846_voidless_return_reject.c
+++ b/test/wcc/846_voidless_return_reject.c
@@ -45,28 +45,56 @@ outbin(char *dst, size_t n, const char *tmpdir, const char *src)
 }
 
 static int
-build_should_fail(const char *driver, const char *label, const char *src,
-    int i)
+filehas(const char *path, const char *needle)
 {
-	char s[64], tmpdir[64], cmd[1024], bin[128];
+	char buf[8192];
+	FILE *f = fopen(path, "rb");
+	if (!f) return 0;
+	size_t n = fread(buf, 1, sizeof buf - 1, f);
+	fclose(f);
+	buf[n] = '\0';
+	return strstr(buf, needle) != NULL;
+}
+
+/* A reject row passes only when the build FAILS *and* the stage's clean
+ * diagnostic appears; a crash (segfault) wraps as a nonzero "w6c failed"
+ * with no diagnostic, so the substring check distinguishes it (#20).
+ * cstage "return void not assignable to i32" and wwstage "return: not
+ * assignable (void → i32)" share the "not assignable" core. */
+static int
+build_should_fail(const char *driver, const char *label, const char *src,
+    const char *diag, int i)
+{
+	char s[64], tmpdir[64], cmd[1024], bin[128], errf[64];
 	snprintf(s, sizeof s, "/tmp/vret_%d_%d.ww", getpid(), i);
 	snprintf(tmpdir, sizeof tmpdir, "/tmp/vret_%d_d_%d", getpid(), i);
+	snprintf(errf, sizeof errf, "/tmp/vret_%d_e_%d.txt", getpid(), i);
 	FILE *f = fopen(s, "wb");
 	if (!f) return -1;
 	fputs(src, f);
 	fclose(f);
 	mkdir(tmpdir, 0755);
-	snprintf(cmd, sizeof cmd, "cd %s && %s build %s 2>/dev/null",
-	    tmpdir, driver, s);
+	snprintf(cmd, sizeof cmd, "cd %s && %s build %s >/dev/null 2>%s",
+	    tmpdir, driver, s, errf);
 	int rc = runwait(cmd);
+	int hasmsg = filehas(errf, diag);
 	outbin(bin, sizeof bin, tmpdir, s);
 	unlink(s);
 	unlink(bin);
+	unlink(errf);
 	rmdir(tmpdir);
-	if (rc == 0)
+	if (rc == 0) {
 		fprintf(stderr, "vret[%s][%s]: built ok, expected a reject\n",
 		    driver, label);
-	return rc == 0 ? -1 : 0;
+		return -1;
+	}
+	if (!hasmsg) {
+		fprintf(stderr, "vret[%s][%s]: nonzero exit but missing "
+		    "diagnostic \"%s\" (a crash, not a clean reject)\n",
+		    driver, label, diag);
+		return -1;
+	}
+	return 0;
 }
 
 static int
@@ -94,13 +122,14 @@ run_build(const char *driver, const char *label, const char *src, int i)
 	return got;
 }
 
-struct rejrow { const char *label; const char *src; };
+struct rejrow { const char *label; const char *src; const char *diag; };
 
 static const struct rejrow reject_rows[] = {
 	{ "nonvoid",
 	  "package main;\n"
 	  "fn f() i32 = { return; };\n"
-	  "fn main() i32 = { return f(); };\n" },
+	  "fn main() i32 = { return f(); };\n",
+	  "not assignable" },
 };
 
 struct okrow { const char *label; const char *src; int want; };
@@ -160,7 +189,7 @@ main(void)
 			total++;
 			if (build_should_fail(drivers[d].path,
 			    reject_rows[i].label, reject_rows[i].src,
-			    d * 100 + i) != 0)
+			    reject_rows[i].diag, d * 100 + i) != 0)
 				fail++;
 		}
 		for (int i = 0; i < nok; i++) {
diff --git a/test/wcc/847_const_reassign_reject.c b/test/wcc/847_const_reassign_reject.c
index bba612e8..87378049 100644
--- a/test/wcc/847_const_reassign_reject.c
+++ b/test/wcc/847_const_reassign_reject.c
@@ -46,28 +46,56 @@ outbin(char *dst, size_t n, const char *tmpdir, const char *src)
 }
 
 static int
-build_should_fail(const char *driver, const char *label, const char *src,
-    int i)
+filehas(const char *path, const char *needle)
 {
-	char s[64], tmpdir[64], cmd[1024], bin[128];
+	char buf[8192];
+	FILE *f = fopen(path, "rb");
+	if (!f) return 0;
+	size_t n = fread(buf, 1, sizeof buf - 1, f);
+	fclose(f);
+	buf[n] = '\0';
+	return strstr(buf, needle) != NULL;
+}
+
+/* A reject row passes only when the build FAILS *and* the stage's clean
+ * diagnostic appears; a crash (segfault) wraps as a nonzero "w6c failed"
+ * with no diagnostic, so the substring check distinguishes it (#20).
+ * cstage "cannot assign to const `x`" and wwstage "cannot assign to const
+ * binding" share the "cannot assign to const" core. */
+static int
+build_should_fail(const char *driver, const char *label, const char *src,
+    const char *diag, int i)
+{
+	char s[64], tmpdir[64], cmd[1024], bin[128], errf[64];
 	snprintf(s, sizeof s, "/tmp/creas_%d_%d.ww", getpid(), i);
 	snprintf(tmpdir, sizeof tmpdir, "/tmp/creas_%d_d_%d", getpid(), i);
+	snprintf(errf, sizeof errf, "/tmp/creas_%d_e_%d.txt", getpid(), i);
 	FILE *f = fopen(s, "wb");
 	if (!f) return -1;
 	fputs(src, f);
 	fclose(f);
 	mkdir(tmpdir, 0755);
-	snprintf(cmd, sizeof cmd, "cd %s && %s build %s 2>/dev/null",
-	    tmpdir, driver, s);
+	snprintf(cmd, sizeof cmd, "cd %s && %s build %s >/dev/null 2>%s",
+	    tmpdir, driver, s, errf);
 	int rc = runwait(cmd);
+	int hasmsg = filehas(errf, diag);
 	outbin(bin, sizeof bin, tmpdir, s);
 	unlink(s);
 	unlink(bin);
+	unlink(errf);
 	rmdir(tmpdir);
-	if (rc == 0)
+	if (rc == 0) {
 		fprintf(stderr, "creas[%s][%s]: built ok, expected a reject\n",
 		    driver, label);
-	return rc == 0 ? -1 : 0;
+		return -1;
+	}
+	if (!hasmsg) {
+		fprintf(stderr, "creas[%s][%s]: nonzero exit but missing "
+		    "diagnostic \"%s\" (a crash, not a clean reject)\n",
+		    driver, label, diag);
+		return -1;
+	}
+	return 0;
 }
 
 static int
@@ -95,12 +123,13 @@ run_build(const char *driver, const char *label, const char *src, int i)
 	return got;
 }
 
-struct rejrow { const char *label; const char *src; };
+struct rejrow { const char *label; const char *src; const char *diag; };
 
 static const struct rejrow reject_rows[] = {
 	{ "const_set",
 	  "package main;\n"
-	  "fn main() i32 = { const x: i32 = 5; x = 6; return x; };\n" },
+	  "fn main() i32 = { const x: i32 = 5; x = 6; return x; };\n",
+	  "cannot assign to const" },
 };
 
 struct okrow { const char *label; const char *src; int want; };
@@ -148,7 +177,7 @@ main(void)
 			total++;
 			if (build_should_fail(drivers[d].path,
 			    reject_rows[i].label, reject_rows[i].src,
-			    d * 100 + i) != 0)
+			    reject_rows[i].diag, d * 100 + i) != 0)
 				fail++;
 		}
 		for (int i = 0; i < nok; i++) {
diff --git a/test/wcc/849_dupfield_reject.c b/test/wcc/849_dupfield_reject.c
index de89a7fb..137ee26c 100644
--- a/test/wcc/849_dupfield_reject.c
+++ b/test/wcc/849_dupfield_reject.c
@@ -49,28 +49,54 @@ outbin(char *dst, size_t n, const char *tmpdir, const char *src)
 }
 
 static int
-build_should_fail(const char *driver, const char *label, const char *src,
-    int i)
+filehas(const char *path, const char *needle)
 {
-	char s[64], tmpdir[64], cmd[1024], bin[128];
+	char buf[8192];
+	FILE *f = fopen(path, "rb");
+	if (!f) return 0;
+	size_t n = fread(buf, 1, sizeof buf - 1, f);
+	fclose(f);
+	buf[n] = '\0';
+	return strstr(buf, needle) != NULL;
+}
+
+/* A reject row passes only when the build FAILS *and* the stage's clean
+ * diagnostic appears; a crash (segfault) wraps as a nonzero "w6c failed"
+ * with no diagnostic, so the substring check distinguishes it (#20). */
+static int
+build_should_fail(const char *driver, const char *label, const char *src,
+    const char *diag, int i)
+{
+	char s[64], tmpdir[64], cmd[1024], bin[128], errf[64];
 	snprintf(s, sizeof s, "/tmp/dupf_%d_%d.ww", getpid(), i);
 	snprintf(tmpdir, sizeof tmpdir, "/tmp/dupf_%d_d_%d", getpid(), i);
+	snprintf(errf, sizeof errf, "/tmp/dupf_%d_e_%d.txt", getpid(), i);
 	FILE *f = fopen(s, "wb");
 	if (!f) return -1;
 	fputs(src, f);
 	fclose(f);
 	mkdir(tmpdir, 0755);
-	snprintf(cmd, sizeof cmd, "cd %s && %s build %s 2>/dev/null",
-	    tmpdir, driver, s);
+	snprintf(cmd, sizeof cmd, "cd %s && %s build %s >/dev/null 2>%s",
+	    tmpdir, driver, s, errf);
 	int rc = runwait(cmd);
+	int hasmsg = filehas(errf, diag);
 	outbin(bin, sizeof bin, tmpdir, s);
 	unlink(s);
 	unlink(bin);
+	unlink(errf);
 	rmdir(tmpdir);
-	if (rc == 0)
+	if (rc == 0) {
 		fprintf(stderr, "dupf[%s][%s]: built ok, expected a reject\n",
 		    driver, label);
-	return rc == 0 ? -1 : 0;
+		return -1;
+	}
+	if (!hasmsg) {
+		fprintf(stderr, "dupf[%s][%s]: nonzero exit but missing "
+		    "diagnostic \"%s\" (a crash, not a clean reject)\n",
+		    driver, label, diag);
+		return -1;
+	}
+	return 0;
 }
 
 static int
@@ -98,21 +124,24 @@ run_build(const char *driver, const char *label, const char *src, int i)
 	return got;
 }
 
-struct rejrow { const char *label; const char *src; };
+struct rejrow { const char *label; const char *src; const char *diag; };
 
 static const struct rejrow reject_rows[] = {
 	{ "adj_dup",
 	  "package main;\n"
 	  "type d = struct { a: i32, a: i32 };\n"
-	  "fn main() i32 = { let p: *d = nil; return 0; };\n" },
+	  "fn main() i32 = { let p: *d = nil; return 0; };\n",
+	  "duplicate field" },
 	{ "nonadj_dup",
 	  "package main;\n"
 	  "type d = struct { a: i32, b: i32, a: i32 };\n"
-	  "fn main() i32 = { let p: *d = nil; return 0; };\n" },
+	  "fn main() i32 = { let p: *d = nil; return 0; };\n",
+	  "duplicate field" },
 	{ "dup_difftype",
 	  "package main;\n"
 	  "type d = struct { a: i32, a: bool };\n"
-	  "fn main() i32 = { let p: *d = nil; return 0; };\n" },
+	  "fn main() i32 = { let p: *d = nil; return 0; };\n",
+	  "duplicate field" },
 };
 
 struct okrow { const char *label; const char *src; int want; };
@@ -162,7 +191,7 @@ main(void)
 			total++;
 			if (build_should_fail(drivers[d].path,
 			    reject_rows[i].label, reject_rows[i].src,
-			    d * 100 + i) != 0)
+			    reject_rows[i].diag, d * 100 + i) != 0)
 				fail++;
 		}
 		for (int i = 0; i < nok; i++) {
diff --git a/test/wcc/850_enum_reject.c b/test/wcc/850_enum_reject.c
index 2b51d99b..e094f305 100644
--- a/test/wcc/850_enum_reject.c
+++ b/test/wcc/850_enum_reject.c
@@ -51,28 +51,54 @@ outbin(char *dst, size_t n, const char *tmpdir, const char *src)
 }
 
 static int
-build_should_fail(const char *driver, const char *label, const char *src,
-    int i)
+filehas(const char *path, const char *needle)
 {
-	char s[64], tmpdir[64], cmd[1024], bin[128];
+	char buf[8192];
+	FILE *f = fopen(path, "rb");
+	if (!f) return 0;
+	size_t n = fread(buf, 1, sizeof buf - 1, f);
+	fclose(f);
+	buf[n] = '\0';
+	return strstr(buf, needle) != NULL;
+}
+
+/* A reject row passes only when the build FAILS *and* the stage's clean
+ * diagnostic appears; a crash (segfault) wraps as a nonzero "w6c failed"
+ * with no diagnostic, so the substring check distinguishes it (#20). */
+static int
+build_should_fail(const char *driver, const char *label, const char *src,
+    const char *diag, int i)
+{
+	char s[64], tmpdir[64], cmd[1024], bin[128], errf[64];
 	snprintf(s, sizeof s, "/tmp/enr_%d_%d.ww", getpid(), i);
 	snprintf(tmpdir, sizeof tmpdir, "/tmp/enr_%d_d_%d", getpid(), i);
+	snprintf(errf, sizeof errf, "/tmp/enr_%d_e_%d.txt", getpid(), i);
 	FILE *f = fopen(s, "wb");
 	if (!f) return -1;
 	fputs(src, f);
 	fclose(f);
 	mkdir(tmpdir, 0755);
-	snprintf(cmd, sizeof cmd, "cd %s && %s build %s 2>/dev/null",
-	    tmpdir, driver, s);
+	snprintf(cmd, sizeof cmd, "cd %s && %s build %s >/dev/null 2>%s",
+	    tmpdir, driver, s, errf);
 	int rc = runwait(cmd);
+	int hasmsg = filehas(errf, diag);
 	outbin(bin, sizeof bin, tmpdir, s);
 	unlink(s);
 	unlink(bin);
+	unlink(errf);
 	rmdir(tmpdir);
-	if (rc == 0)
+	if (rc == 0) {
 		fprintf(stderr, "enr[%s][%s]: built ok, expected a reject\n",
 		    driver, label);
-	return rc == 0 ? -1 : 0;
+		return -1;
+	}
+	if (!hasmsg) {
+		fprintf(stderr, "enr[%s][%s]: nonzero exit but missing "
+		    "diagnostic \"%s\" (a crash, not a clean reject)\n",
+		    driver, label, diag);
+		return -1;
+	}
+	return 0;
 }
 
 static int
@@ -100,21 +126,28 @@ run_build(const char *driver, const char *label, const char *src, int i)
 	return got;
 }
 
-struct rejrow { const char *label; const char *src; };
+struct rejrow { const char *label; const char *src; const char *diag; };
 
+/* diag substrings are the shared message BODY (no file:line prefix, which
+ * differs cstage vs wwstage). fwd_ref's stages diverge in wording (cstage
+ * "enum value: unknown identifier 'B'" vs wwstage "enum value must be a
+ * constant integer expression"); "enum value" is the common core (#20). */
 static const struct rejrow reject_rows[] = {
 	{ "nonint_stor",
 	  "package main;\n"
 	  "type e = enum f64 { A };\n"
-	  "fn main() i32 = { return e.A as i32; };\n" },
+	  "fn main() i32 = { return e.A as i32; };\n",
+	  "enum storage type must be integer" },
 	{ "dup_member",
 	  "package main;\n"
 	  "type e = enum { A, B, A };\n"
-	  "fn main() i32 = { return e.A as i32; };\n" },
+	  "fn main() i32 = { return e.A as i32; };\n",
+	  "duplicate enum member" },
 	{ "fwd_ref",
 	  "package main;\n"
 	  "type e = enum { A = B, B };\n"
-	  "fn main() i32 = { return e.A as i32; };\n" },
+	  "fn main() i32 = { return e.A as i32; };\n",
+	  "enum value" },
 };
 
 struct okrow { const char *label; const char *src; int want; };
@@ -164,7 +197,7 @@ main(void)
 			total++;
 			if (build_should_fail(drivers[d].path,
 			    reject_rows[i].label, reject_rows[i].src,
-			    d * 100 + i) != 0)
+			    reject_rows[i].diag, d * 100 + i) != 0)
 				fail++;
 		}
 		for (int i = 0; i < nok; i++) {
diff --git a/test/wcc/851_index_int_reject.c b/test/wcc/851_index_int_reject.c
index 2506332d..3abafeb1 100644
--- a/test/wcc/851_index_int_reject.c
+++ b/test/wcc/851_index_int_reject.c
@@ -60,28 +60,54 @@ outbin(char *dst, size_t n, const char *tmpdir, const char *src)
 }
 
 static int
-build_should_fail(const char *driver, const char *label, const char *src,
-    int i)
+filehas(const char *path, const char *needle)
 {
-	char s[64], tmpdir[64], cmd[1024], bin[128];
+	char buf[8192];
+	FILE *f = fopen(path, "rb");
+	if (!f) return 0;
+	size_t n = fread(buf, 1, sizeof buf - 1, f);
+	fclose(f);
+	buf[n] = '\0';
+	return strstr(buf, needle) != NULL;
+}
+
+/* A reject row passes only when the build FAILS *and* the stage's clean
+ * diagnostic appears; a crash (segfault) wraps as a nonzero "w6c failed"
+ * with no diagnostic, so the substring check distinguishes it (#20). */
+static int
+build_should_fail(const char *driver, const char *label, const char *src,
+    const char *diag, int i)
+{
+	char s[64], tmpdir[64], cmd[1024], bin[128], errf[64];
 	snprintf(s, sizeof s, "/tmp/idxr_%d_%d.ww", getpid(), i);
 	snprintf(tmpdir, sizeof tmpdir, "/tmp/idxr_%d_d_%d", getpid(), i);
+	snprintf(errf, sizeof errf, "/tmp/idxr_%d_e_%d.txt", getpid(), i);
 	FILE *f = fopen(s, "wb");
 	if (!f) return -1;
 	fputs(src, f);
 	fclose(f);
 	mkdir(tmpdir, 0755);
-	snprintf(cmd, sizeof cmd, "cd %s && %s build %s 2>/dev/null",
-	    tmpdir, driver, s);
+	snprintf(cmd, sizeof cmd, "cd %s && %s build %s >/dev/null 2>%s",
+	    tmpdir, driver, s, errf);
 	int rc = runwait(cmd);
+	int hasmsg = filehas(errf, diag);
 	outbin(bin, sizeof bin, tmpdir, s);
 	unlink(s);
 	unlink(bin);
+	unlink(errf);
 	rmdir(tmpdir);
-	if (rc == 0)
+	if (rc == 0) {
 		fprintf(stderr, "idxr[%s][%s]: built ok, expected a reject\n",
 		    driver, label);
-	return rc == 0 ? -1 : 0;
+		return -1;
+	}
+	if (!hasmsg) {
+		fprintf(stderr, "idxr[%s][%s]: nonzero exit but missing "
+		    "diagnostic \"%s\" (a crash, not a clean reject)\n",
+		    driver, label, diag);
+		return -1;
+	}
+	return 0;
 }
 
 static int
@@ -109,7 +135,7 @@ run_build(const char *driver, const char *label, const char *src, int i)
 	return got;
 }
 
-struct rejrow { const char *label; const char *src; };
+struct rejrow { const char *label; const char *src; const char *diag; };
 
 static const struct rejrow reject_rows[] = {
 	{ "by_float",
@@ -118,28 +144,32 @@ static const struct rejrow reject_rows[] = {
 	  "	let a: [3]i32 = [10i32, 20i32, 30i32];\n"
 	  "	let f: f64 = 1.0;\n"
 	  "	return a[f];\n"
-	  "};\n" },
+	  "};\n",
+	  "index must be integer" },
 	{ "by_str",
 	  "package main;\n"
 	  "fn main() i32 = {\n"
 	  "	let a: [3]i32 = [10i32, 20i32, 30i32];\n"
 	  "	let s: str = \"x\";\n"
 	  "	return a[s];\n"
-	  "};\n" },
+	  "};\n",
+	  "index must be integer" },
 	{ "by_ptr",
 	  "package main;\n"
 	  "fn main() i32 = {\n"
 	  "	let a: [3]i32 = [10i32, 20i32, 30i32];\n"
 	  "	let p: *i32 = nil;\n"
 	  "	return a[p];\n"
-	  "};\n" },
+	  "};\n",
+	  "index must be integer" },
 	{ "by_bool",
 	  "package main;\n"
 	  "fn main() i32 = {\n"
 	  "	let a: [3]i32 = [10i32, 20i32, 30i32];\n"
 	  "	let b: bool = true;\n"
 	  "	return a[b];\n"
-	  "};\n" },
+	  "};\n",
+	  "index must be integer" },
 };
 
 struct okrow { const char *label; const char *src; int want; };
@@ -217,7 +247,7 @@ main(void)
 			total++;
 			if (build_should_fail(drivers[d].path,
 			    reject_rows[i].label, reject_rows[i].src,
-			    d * 100 + i) != 0)
+			    reject_rows[i].diag, d * 100 + i) != 0)
 				fail++;
 		}
 		for (int i = 0; i < nok; i++) {
diff --git a/test/wcc/989_callarg_typecheck.c b/test/wcc/989_callarg_typecheck.c
index 56a65f88..22dbf9c5 100644
--- a/test/wcc/989_callarg_typecheck.c
+++ b/test/wcc/989_callarg_typecheck.c
@@ -68,6 +68,7 @@ struct row {
 	const char *src;
 	int expect_build;	/* 1 = build+run to want_exit; 0 = must FAIL */
 	int want_exit;		/* meaningful only when expect_build */
+	const char *diag;	/* expected reject diagnostic (NULL for build rows) */
 };
 
 static const struct row rows[] = {
@@ -81,7 +82,7 @@ static const struct row rows[] = {
 	  "	takesslice(n);\n"
 	  "	return 0;\n"
 	  "};\n",
-	  0, 0 },
+	  0, 0, "not assignable" },
 
 	/* (2) str where int is wanted → REJECT both stages. A second scalar
 	 * shape: the general check is not array-specific. */
@@ -93,7 +94,7 @@ static const struct row rows[] = {
 	  "	takesint(s);\n"
 	  "	return 0;\n"
 	  "};\n",
-	  0, 0 },
+	  0, 0, "not assignable" },
 
 	/* (3) CONTROL — [3]int into a []int param is the legit #258 borrow
 	 * (matching element) → ACCEPT both, run to 0. The general check must
@@ -122,7 +123,7 @@ static const struct row rows[] = {
 	{ "let_slice_eq_scalar",
 	  "package main;\n"
 	  "export fn main() int = { let xs: []int = 5; return 0; };\n",
-	  0, 0 },
+	  0, 0, "not assignable" },
 
 	/* (6) bare fn name into a fn-alias param → ACCEPT both (#34/c1': a
 	 * bare fn rvalue types as its fn TYPE; the general check then sees a
@@ -154,7 +155,7 @@ static const struct row rows[] = {
 	  "fn g() int = { return 7; };\n"
 	  "fn use_it(f: myfn) int = { return f(); };\n"
 	  "export fn main() int = { return use_it(&g); };\n",
-	  0, 0 },
+	  0, 0, "not assignable" },
 
 	/* (9) matched-signature fn rvalue into a fn slot → ACCEPT both, run 5.
 	 * Pins #34's correct-stamp path (fn type vs fn type, equal sigs). */
@@ -172,7 +173,7 @@ static const struct row rows[] = {
 	  "package main;\n"
 	  "fn h() str = { return \"\"; };\n"
 	  "export fn main() int = { let p: fn() int = h; return 0; };\n",
-	  0, 0 },
+	  0, 0, "not assignable" },
 
 	/* (11) aggregate<->aggregate KIND mismatch — a [3]int array arg into a
 	 * `*int` param. Both aggregate, different kind -> confident reject (the
@@ -186,7 +187,7 @@ static const struct row rows[] = {
 	  "	takesptr(a);\n"
 	  "	return 0;\n"
 	  "};\n",
-	  0, 0 },
+	  0, 0, "not assignable" },
 
 	/* (12) CONTROL — a concrete value into a SPREAD-tagged param must NOT
 	 * over-reject: cstage flattens `...inner` at resolve_type and accepts
@@ -234,14 +235,31 @@ run_build(const char *driver, const struct row *r, int i)
 	return brc == 0 ? got : -1;
 }
 
-/* build_should_fail — the build must error on `driver`; returns 0 when
- * it correctly FAILS, non-zero when it wrongly succeeded. */
 static int
-build_should_fail(const char *driver, const char *src, int i)
+filehas(const char *path, const char *needle)
 {
-	char s[64], tmpdir[64], cmd[1024];
+	char buf[8192];
+	FILE *f = fopen(path, "rb");
+	if (!f) return 0;
+	size_t n = fread(buf, 1, sizeof buf - 1, f);
+	fclose(f);
+	buf[n] = '\0';
+	return strstr(buf, needle) != NULL;
+}
+
+/* build_should_fail — the build must FAIL on `driver` *and* emit `diag`.
+ * A crash (segfault) wraps as a nonzero "w6c failed" with no diagnostic,
+ * so the substring check distinguishes it from a clean reject (#20). All
+ * rows[] reject on "not assignable" (cstage spells the types, wwstage the
+ * shorter "ctx: not assignable (...)"); the core is shared. */
+static int
+build_should_fail(const char *driver, const char *src, const char *diag,
+    int i)
+{
+	char s[64], tmpdir[64], cmd[1024], errf[64];
 	snprintf(s, sizeof s, "/tmp/catn_%d_%d.ww", getpid(), i);
 	snprintf(tmpdir, sizeof tmpdir, "/tmp/catn_%d_d_%d", getpid(), i);
+	snprintf(errf, sizeof errf, "/tmp/catn_%d_e_%d.txt", getpid(), i);
 
 	FILE *f = fopen(s, "wb");
 	if (!f) return -1;
@@ -249,11 +267,13 @@ build_should_fail(const char *driver, const char *src, int i)
 	fclose(f);
 
 	mkdir(tmpdir, 0755);
-	snprintf(cmd, sizeof cmd, "cd %s && %s build %s 2>/dev/null",
-	    tmpdir, driver, s);
+	snprintf(cmd, sizeof cmd, "cd %s && %s build %s >/dev/null 2>%s",
+	    tmpdir, driver, s, errf);
 	int rc = runwait(cmd);
+	int hasmsg = filehas(errf, diag);
 
 	unlink(s);
+	unlink(errf);
 	const char *base = strrchr(s, '/');
 	base = base ? base + 1 : s;
 	char outbin[128];
@@ -262,7 +282,8 @@ build_should_fail(const char *driver, const char *src, int i)
 	if (dot && strcmp(dot, ".ww") == 0) *dot = '\0';
 	unlink(outbin);
 	rmdir(tmpdir);
-	return rc == 0 ? -1 : 0;	/* build must NOT succeed */
+	/* build must NOT succeed AND must emit its diagnostic. */
+	return (rc != 0 && hasmsg) ? 0 : -1;
 }
 
 /* -T reject faces — a user fixture is bundled with lib/test via
@@ -313,12 +334,17 @@ tbundle_reject(const char *bin, const char *stage, const char *drv,
 	fputs(t->src, f);
 	fclose(f);
 
+	char errf[176];
+	snprintf(errf, sizeof errf, "%s.err", stem);
 	snprintf(cmd, sizeof cmd,
-	    "WW_PKGCACHE=%s.pkgc %s/%s test --sep -o %s %s > /dev/null 2>&1",
-	    stem, bin, drv, stem, src);
+	    "WW_PKGCACHE=%s.pkgc %s/%s test --sep -o %s %s >/dev/null 2>%s",
+	    stem, bin, drv, stem, src, errf);
 	int rc = runwait(cmd);
+	/* Both stages emit the reservation diagnostic; a crash would not (#20). */
+	int hasmsg = filehas(errf, "__wwtests is reserved by -T");
 
 	unlink(src);
+	unlink(errf);
 	snprintf(cmd, sizeof cmd, "rm -rf %s %s.sepwork %s.pkgc",
 	    stem, stem, stem);
 	if (system(cmd)) {}
@@ -327,6 +353,12 @@ tbundle_reject(const char *bin, const char *stage, const char *drv,
 		    "accepted (expected a loud reject)\n", stage, t->label, drv);
 		return -1;
 	}
+	if (!hasmsg) {
+		fprintf(stderr, "callarg_typecheck[%s][%s]: %s test --sep "
+		    "nonzero exit but missing reservation diagnostic (a crash, "
+		    "not a clean reject)\n", stage, t->label, drv);
+		return -1;
+	}
 	return 0;
 }
 
@@ -375,20 +407,25 @@ multimod_build_fail(const char *drv, const char *mainbody, int withmod1,
 	fputs(mainbody, f);
 	fclose(f);
 
+	char errf[128];
+	snprintf(errf, sizeof errf, "%s/err.txt", dir);
 	if (withmod1)
 		snprintf(cmd, sizeof cmd,
-		    "cd %s && %s build -I %s -I %s main.ww >/dev/null 2>&1",
-		    dir, drv, io2d, mod1d);
+		    "cd %s && %s build -I %s -I %s main.ww >/dev/null 2>%s",
+		    dir, drv, io2d, mod1d, errf);
 	else
 		snprintf(cmd, sizeof cmd,
-		    "cd %s && %s build -I %s main.ww >/dev/null 2>&1",
-		    dir, drv, io2d);
+		    "cd %s && %s build -I %s main.ww >/dev/null 2>%s",
+		    dir, drv, io2d, errf);
 	int rc = runwait(cmd);
+	/* Both collision rows reject on "not assignable"; a crash would not (#20). */
+	int hasmsg = filehas(errf, "not assignable");
 
 	snprintf(rm, sizeof rm, "rm -rf %s", dir);
 	runwait(rm);
 
-	return rc == 0 ? -1 : 0;	/* build must NOT succeed */
+	/* build must NOT succeed AND must emit its diagnostic. */
+	return (rc != 0 && hasmsg) ? 0 : -1;
 }
 
 /* a []int SLICE passed where io2.handle = (file | stream) is wanted — NO
@@ -468,7 +505,7 @@ main(void)
 				}
 			} else {
 				if (build_should_fail(drivers[d].drv, rows[i].src,
-				    100 + i) != 0) {
+				    rows[i].diag, 100 + i) != 0) {
 					fprintf(stderr, "callarg_typecheck[%s][%s]: "
 					    "built ok, expected a loud reject\n",
 					    drivers[d].name, rows[i].label);
diff --git a/test/wcc/989_catA_f2_reject.c b/test/wcc/989_catA_f2_reject.c
index fc0bb6d0..8148c002 100644
--- a/test/wcc/989_catA_f2_reject.c
+++ b/test/wcc/989_catA_f2_reject.c
@@ -40,6 +40,8 @@ struct row {
 	const char *src;
 	int expect_build;	/* 1 = build+run to want_exit; 0 = must FAIL */
 	int want_exit;
+	const char *diag;	/* cstage/shared reject diagnostic (NULL for build rows) */
+	const char *wdiag;	/* wwstage override when stages diverge (NULL = use diag) */
 };
 
 static const struct row rows[] = {
@@ -53,7 +55,7 @@ static const struct row rows[] = {
 	  "	xs[0] = 77u8;\n"
 	  "	return n;\n"
 	  "};\n",
-	  0, 0 },
+	  0, 0, "array length must be an integer literal" },
 
 	/* #2 control — a literal dimension stays valid → run 0. */
 	{ "arrlen_const_ok",
@@ -75,7 +77,7 @@ static const struct row rows[] = {
 	  "	let c: str = a + b;\n"
 	  "	return c.len: i32;\n"
 	  "};\n",
-	  0, 0 },
+	  0, 0, "arithmetic on non-numeric type" },
 
 	/* #6 — bitwise & on str is non-integer → REJECT. */
 	{ "str_bitand",
@@ -86,7 +88,7 @@ static const struct row rows[] = {
 	  "	let n: i32 = (a & b): i32;\n"
 	  "	return n;\n"
 	  "};\n",
-	  0, 0 },
+	  0, 0, "bitwise on non-integer type" },
 
 	/* #6 — ordered comparison on str is non-numeric → REJECT (EQ/NEQ stay
 	 * valid; only the ordered <,<=,>,>= are gated, matching cstage). */
@@ -98,7 +100,7 @@ static const struct row rows[] = {
 	  "	if (a < b) { return 1; };\n"
 	  "	return 0;\n"
 	  "};\n",
-	  0, 0 },
+	  0, 0, "ordered comparison on non-numeric" },
 
 	/* #6 — unary ~ on a non-integer (str) → REJECT. */
 	{ "tilde_str",
@@ -108,7 +110,7 @@ static const struct row rows[] = {
 	  "	let n: i32 = (~a): i32;\n"
 	  "	return n;\n"
 	  "};\n",
-	  0, 0 },
+	  0, 0, "~ on non-integer" },
 
 	/* #6 control — integer arithmetic, comparison, bitwise all valid. */
 	{ "int_ops_ok",
@@ -130,7 +132,7 @@ static const struct row rows[] = {
 	  "	let v: i32 = 5;\n"
 	  "	return size(v): i32;\n"
 	  "};\n",
-	  0, 0 },
+	  0, 0, "unknown type 'v'" },
 
 	/* #7 control — size() of a real type folds → run 4. */
 	{ "size_type_ok",
@@ -145,7 +147,7 @@ static const struct row rows[] = {
 	  "	let t: (i32, i32) = (1, 2, 3);\n"
 	  "	return 0;\n"
 	  "};\n",
-	  0, 0 },
+	  0, 0, "not assignable to declared", "tuple literal has 3 elements" },
 
 	/* #10 — a short tuple literal → REJECT (the live miscompile: cgen
 	 * filled the missing slot with a stale register). */
@@ -155,7 +157,7 @@ static const struct row rows[] = {
 	  "	let t: (i32, i32, i32) = (1, 2);\n"
 	  "	return 0;\n"
 	  "};\n",
-	  0, 0 },
+	  0, 0, "not assignable to declared", "tuple literal has 2 elements" },
 
 	/* #10 control — a matching-arity tuple literal → run 3. */
 	{ "tuple_match_ok",
@@ -178,7 +180,7 @@ static const struct row rows[] = {
 	  "	a, s = f();\n"
 	  "	return 0;\n"
 	  "};\n",
-	  0, 0 },
+	  0, 0, "multi-assign rhs is not a tuple" },
 
 	/* #39 — multi-assign rhs is a plain scalar, not a tuple → REJECT. */
 	{ "massign_scalar",
@@ -190,7 +192,7 @@ static const struct row rows[] = {
 	  "	a, s = g();\n"
 	  "	return 0;\n"
 	  "};\n",
-	  0, 0 },
+	  0, 0, "multi-assign rhs is not a tuple" },
 
 	/* #39 control — a bare-tuple-returning fn drives a valid massign. */
 	{ "massign_tuple_ok",
@@ -213,7 +215,7 @@ static const struct row rows[] = {
 	  "type e = !void;\n"
 	  "fn g() (...inner | e) = { return true; };\n"
 	  "export fn main() i32 = { let x = g()!; return 0; };\n",
-	  0, 0 },
+	  0, 0, "multi-success union unwired" },
 
 	/* #8 control — a single-success tagged union try unwraps fine → run 5. */
 	{ "try_single_ok",
@@ -234,7 +236,7 @@ static const struct row rows[] = {
 	  "	let r = match (v) { case i32 => yield 7; case str => yield \"oops\"; };\n"
 	  "	return 0;\n"
 	  "};\n",
-	  0, 0 },
+	  0, 0, "match arm yields" },
 
 	/* match-yield control — same-family arm yields (untyped_int + i32) must
 	 * NOT over-reject (the untyped->concrete promotion cstage admits) → run 9. */
@@ -281,11 +283,28 @@ run_build(const char *driver, const struct row *r, int i)
 }
 
 static int
-build_should_fail(const char *driver, const char *src, int i)
+filehas(const char *path, const char *needle)
 {
-	char s[64], tmpdir[64], cmd[1024];
+	char buf[8192];
+	FILE *f = fopen(path, "rb");
+	if (!f) return 0;
+	size_t n = fread(buf, 1, sizeof buf - 1, f);
+	fclose(f);
+	buf[n] = '\0';
+	return strstr(buf, needle) != NULL;
+}
+
+/* build_should_fail — the build must FAIL *and* emit `diag`. A crash
+ * (segfault) wraps as a nonzero "w6c failed" with no diagnostic, so the
+ * substring check distinguishes it from a clean reject (#20). */
+static int
+build_should_fail(const char *driver, const char *src, const char *diag,
+    int i)
+{
+	char s[64], tmpdir[64], cmd[1024], errf[64];
 	snprintf(s, sizeof s, "/tmp/f2n_%d_%d.ww", getpid(), i);
 	snprintf(tmpdir, sizeof tmpdir, "/tmp/f2n_%d_d_%d", getpid(), i);
+	snprintf(errf, sizeof errf, "/tmp/f2n_%d_e_%d.txt", getpid(), i);
 
 	FILE *f = fopen(s, "wb");
 	if (!f) return -1;
@@ -293,11 +312,13 @@ build_should_fail(const char *driver, const char *src, int i)
 	fclose(f);
 
 	mkdir(tmpdir, 0755);
-	snprintf(cmd, sizeof cmd, "cd %s && %s build %s 2>/dev/null",
-	    tmpdir, driver, s);
+	snprintf(cmd, sizeof cmd, "cd %s && %s build %s >/dev/null 2>%s",
+	    tmpdir, driver, s, errf);
 	int rc = runwait(cmd);
+	int hasmsg = filehas(errf, diag);
 
 	unlink(s);
+	unlink(errf);
 	const char *base = strrchr(s, '/');
 	base = base ? base + 1 : s;
 	char outbin[128];
@@ -306,7 +327,8 @@ build_should_fail(const char *driver, const char *src, int i)
 	if (dot && strcmp(dot, ".ww") == 0) *dot = '\0';
 	unlink(outbin);
 	rmdir(tmpdir);
-	return rc == 0 ? -1 : 0;	/* build must NOT succeed */
+	/* build must NOT succeed AND must emit its diagnostic. */
+	return (rc != 0 && hasmsg) ? 0 : -1;
 }
 
 int
@@ -354,6 +376,8 @@ main(void)
 				}
 			} else {
 				if (build_should_fail(drivers[d].drv, rows[i].src,
+				    drivers[d].gated && rows[i].wdiag
+				    ? rows[i].wdiag : rows[i].diag,
 				    100 + i) != 0) {
 					fprintf(stderr, "catA_f2_reject[%s][%s]: "
 					    "built ok, expected a loud reject\n",
diff --git a/test/wcc/989_intoverflow_reject.c b/test/wcc/989_intoverflow_reject.c
index b6d724ed..90a76cbf 100644
--- a/test/wcc/989_intoverflow_reject.c
+++ b/test/wcc/989_intoverflow_reject.c
@@ -39,6 +39,7 @@ struct row {
 	const char *src;
 	int expect_build;	/* 1 = build+run to want_exit; 0 = must FAIL */
 	int want_exit;
+	const char *diag;	/* expected reject diagnostic (NULL for build rows) */
 };
 
 static const struct row rows[] = {
@@ -49,7 +50,7 @@ static const struct row rows[] = {
 	  "	let x: u64 = 99999999999999999999u64;\n"
 	  "	return x: i32;\n"
 	  "};\n",
-	  0, 0 },
+	  0, 0, "bad integer literal" },
 
 	/* REJECT — 65-bit hex (smallest overflow above U64_MAX). */
 	{ "hex_overflow",
@@ -58,7 +59,7 @@ static const struct row rows[] = {
 	  "	let x: u64 = 0x1ffffffffffffffffu64;\n"
 	  "	return x: i32;\n"
 	  "};\n",
-	  0, 0 },
+	  0, 0, "bad integer literal" },
 
 	/* REJECT — 2^128-1 decimal, far over u64. */
 	{ "dec_huge",
@@ -67,7 +68,7 @@ static const struct row rows[] = {
 	  "	let x: u64 = 340282366920938463463374607431768211455u64;\n"
 	  "	return x: i32;\n"
 	  "};\n",
-	  0, 0 },
+	  0, 0, "bad integer literal" },
 
 	/* control — U64_MAX decimal is the largest accepted literal. */
 	{ "u64max_dec_ok",
@@ -122,11 +123,30 @@ run_build(const char *driver, const struct row *r, int i)
 }
 
 static int
-build_should_fail(const char *driver, const char *src, int i)
+filehas(const char *path, const char *needle)
 {
-	char s[64], tmpdir[64], cmd[1024];
+	char buf[8192];
+	FILE *f = fopen(path, "rb");
+	if (!f) return 0;
+	size_t n = fread(buf, 1, sizeof buf - 1, f);
+	fclose(f);
+	buf[n] = '\0';
+	return strstr(buf, needle) != NULL;
+}
+
+/* build_should_fail — the build must FAIL *and* emit `diag`. A crash
+ * (segfault) wraps as a nonzero "w6c failed" with no diagnostic, so the
+ * substring check distinguishes it from a clean reject (#20). cstage
+ * quotes the literal ("bad integer literal '99...'"), wwstage does not;
+ * "bad integer literal" is the shared core. */
+static int
+build_should_fail(const char *driver, const char *src, const char *diag,
+    int i)
+{
+	char s[64], tmpdir[64], cmd[1024], errf[64];
 	snprintf(s, sizeof s, "/tmp/iovn_%d_%d.ww", getpid(), i);
 	snprintf(tmpdir, sizeof tmpdir, "/tmp/iovn_%d_d_%d", getpid(), i);
+	snprintf(errf, sizeof errf, "/tmp/iovn_%d_e_%d.txt", getpid(), i);
 
 	FILE *f = fopen(s, "wb");
 	if (!f) return -1;
@@ -134,11 +154,13 @@ build_should_fail(const char *driver, const char *src, int i)
 	fclose(f);
 
 	mkdir(tmpdir, 0755);
-	snprintf(cmd, sizeof cmd, "cd %s && %s build %s 2>/dev/null",
-	    tmpdir, driver, s);
+	snprintf(cmd, sizeof cmd, "cd %s && %s build %s >/dev/null 2>%s",
+	    tmpdir, driver, s, errf);
 	int rc = runwait(cmd);
+	int hasmsg = filehas(errf, diag);
 
 	unlink(s);
+	unlink(errf);
 	const char *base = strrchr(s, '/');
 	base = base ? base + 1 : s;
 	char outbin[128];
@@ -147,7 +169,8 @@ build_should_fail(const char *driver, const char *src, int i)
 	if (dot && strcmp(dot, ".ww") == 0) *dot = '\0';
 	unlink(outbin);
 	rmdir(tmpdir);
-	return rc == 0 ? -1 : 0;	/* build must NOT succeed */
+	/* build must NOT succeed AND must emit its diagnostic. */
+	return (rc != 0 && hasmsg) ? 0 : -1;
 }
 
 int
@@ -195,7 +218,7 @@ main(void)
 				}
 			} else {
 				if (build_should_fail(drivers[d].drv, rows[i].src,
-				    100 + i) != 0) {
+				    rows[i].diag, 100 + i) != 0) {
 					fprintf(stderr, "intoverflow_reject[%s][%s]: "
 					    "built ok, expected a loud reject\n",
 					    drivers[d].name, rows[i].label);
diff --git a/test/wcc/989_tagged_subset_reject.c b/test/wcc/989_tagged_subset_reject.c
index c467552e..fc877a96 100644
--- a/test/wcc/989_tagged_subset_reject.c
+++ b/test/wcc/989_tagged_subset_reject.c
@@ -89,6 +89,7 @@ struct row {
 	int expect_build;	/* 1 = build+run to want_exit; 0 = must FAIL */
 	int want_exit;		/* meaningful only when expect_build */
 	int byteid;		/* 1 = also assert w6c vs w6c_ww .s byte-id */
+	const char *diag;	/* expected reject diagnostic (NULL for build rows) */
 };
 
 static const struct row rows[] = {
@@ -100,7 +101,7 @@ static const struct row rows[] = {
 	  "type small = (int | bool);\n"
 	  "fn f(b: big) small = { return b; };\n"
 	  "export fn main() int = { return 0; };\n",
-	  0, 0, 0 },
+	  0, 0, 0, "not assignable" },
 
 	/* (2) D flatten-subset, union→union → ACCEPT + byte-id.
 	 * {bool,str} ⊆ {int,bool,str}; cstage flattens inner + remaps tags. */
@@ -172,7 +173,7 @@ static const struct row rows[] = {
 	  "	};\n"
 	  "};\n"
 	  "export fn main() int = { return 0; };\n",
-	  0, 0, 0 },
+	  0, 0, 0, "not a variant" },
 
 	/* (5b) spread → wider subset assign (latent) → KEEP lenient accept.
 	 * sp's src side carries a `...e1` spread → the guard keeps the
@@ -221,14 +222,29 @@ run_build(const char *driver, const struct row *r, int i)
 	return brc == 0 ? got : -1;
 }
 
-/* build_should_fail — the build must error on `driver`; returns 0 when it
- * correctly FAILS, non-zero when it wrongly succeeded. */
 static int
-build_should_fail(const char *driver, const char *src, int i)
+filehas(const char *path, const char *needle)
 {
-	char s[64], tmpdir[64], cmd[1024];
+	char buf[8192];
+	FILE *f = fopen(path, "rb");
+	if (!f) return 0;
+	size_t n = fread(buf, 1, sizeof buf - 1, f);
+	fclose(f);
+	buf[n] = '\0';
+	return strstr(buf, needle) != NULL;
+}
+
+/* build_should_fail — the build must FAIL on `driver` *and* emit `diag`.
+ * A crash (segfault) wraps as a nonzero "w6c failed" with no diagnostic,
+ * so the substring check distinguishes it from a clean reject (#20). */
+static int
+build_should_fail(const char *driver, const char *src, const char *diag,
+    int i)
+{
+	char s[64], tmpdir[64], cmd[1024], errf[64];
 	snprintf(s, sizeof s, "/tmp/tsrn_%d_%d.ww", getpid(), i);
 	snprintf(tmpdir, sizeof tmpdir, "/tmp/tsrn_%d_d_%d", getpid(), i);
+	snprintf(errf, sizeof errf, "/tmp/tsrn_%d_e_%d.txt", getpid(), i);
 
 	FILE *f = fopen(s, "wb");
 	if (!f) return -1;
@@ -236,11 +252,13 @@ build_should_fail(const char *driver, const char *src, int i)
 	fclose(f);
 
 	mkdir(tmpdir, 0755);
-	snprintf(cmd, sizeof cmd, "cd %s && %s build %s 2>/dev/null",
-	    tmpdir, driver, s);
+	snprintf(cmd, sizeof cmd, "cd %s && %s build %s >/dev/null 2>%s",
+	    tmpdir, driver, s, errf);
 	int rc = runwait(cmd);
+	int hasmsg = filehas(errf, diag);
 
 	unlink(s);
+	unlink(errf);
 	const char *base = strrchr(s, '/');
 	base = base ? base + 1 : s;
 	char outbin[128];
@@ -249,7 +267,8 @@ build_should_fail(const char *driver, const char *src, int i)
 	if (dot && strcmp(dot, ".ww") == 0) *dot = '\0';
 	unlink(outbin);
 	rmdir(tmpdir);
-	return rc == 0 ? -1 : 0;	/* build must NOT succeed */
+	/* build must NOT succeed AND must emit its diagnostic. */
+	return (rc != 0 && hasmsg) ? 0 : -1;
 }
 
 /* asm_byte_identical — w6c vs w6c_ww .s for the same source must match. */
@@ -314,6 +333,7 @@ struct xrow {
 	const char *msrc;	/* package main; import e */
 	int expect_build;	/* 1 = build+run to want_exit; 0 = must FAIL */
 	int want_exit;
+	const char *rejdiag;	/* expected reject diagnostic (NULL for build rows) */
 };
 
 static const struct xrow xrows[] = {
@@ -345,7 +365,7 @@ static const struct xrow xrows[] = {
 	  "type myres = (i64 | e.done);\n"
 	  "fn forward() myres = { return e.next(); };\n"
 	  "export fn main() i32 = { return 0; };\n",
-	  0, 0 },
+	  0, 0, "not assignable" },
 };
 
 /* run_xmod — write e.ww + main.ww into a fresh dir, `ww build -I dir
@@ -371,19 +391,26 @@ run_xmod(const char *driver, const struct xrow *x)
 		return -2;
 	}
 
+	char errf[1056];
+	snprintf(errf, sizeof errf, "%s/err.txt", dir);
 	snprintf(cmd, sizeof cmd, "cd %s && %s build -I %s %s/main.ww "
-	    "2>/dev/null", dir, driver, dir, dir);
+	    ">/dev/null 2>%s", dir, driver, dir, dir, errf);
 	int brc = runwait(cmd);
 
 	int got = -1;
 	if (brc == 0) {
 		snprintf(path, sizeof path, "%s/main", dir);
 		got = runwait(path);
+	} else if (x->rejdiag && !filehas(errf, x->rejdiag)) {
+		/* A REJECT row whose build failed WITHOUT its diagnostic is a
+		 * crash, not a clean reject — return a non-(-1) sentinel so the
+		 * caller (which expects -1 on a clean reject) flags it (#20). */
+		got = -3;
 	}
 
 	snprintf(cmd, sizeof cmd, "rm -rf %s", dir);
 	(void)runwait(cmd);
-	return brc == 0 ? got : -1;
+	return got;
 }
 
 int
@@ -434,7 +461,7 @@ main(void)
 				}
 			} else {
 				if (build_should_fail(drivers[d].path,
-				    rows[i].src, 100 + i) != 0) {
+				    rows[i].src, rows[i].diag, 100 + i) != 0) {
 					fprintf(stderr,
 					    "tagged_subset_reject[%s][%s]: "
 					    "built ok, expected a loud reject\n",
diff --git a/test/wcc/989_unknowndecl_reject.c b/test/wcc/989_unknowndecl_reject.c
index 9533c79f..fe4397e9 100644
--- a/test/wcc/989_unknowndecl_reject.c
+++ b/test/wcc/989_unknowndecl_reject.c
@@ -37,6 +37,7 @@ struct row {
 	const char *src;
 	int expect_build;	/* 1 = build+run to want_exit; 0 = must FAIL */
 	int want_exit;
+	const char *diag;	/* expected reject diagnostic (NULL for build rows) */
 };
 
 static const struct row rows[] = {
@@ -46,14 +47,14 @@ static const struct row rows[] = {
 	  "package main;\n"
 	  "fnn helper() i32 = { return 42; };\n"
 	  "export fn main() i32 = { return 0; };\n",
-	  0, 0 },
+	  0, 0, "expected top-level decl" },
 
 	/* REJECT — a stray run of bare identifiers at top level. */
 	{ "stray_idents",
 	  "package main;\n"
 	  "bogus token here;\n"
 	  "export fn main() i32 = { return 0; };\n",
-	  0, 0 },
+	  0, 0, "expected top-level decl" },
 
 	/* REJECT — a stray decl whose body has internal ';'s, exercising the
 	 * brace-balanced chew recovery (still must error, not silently skip). */
@@ -61,7 +62,7 @@ static const struct row rows[] = {
 	  "package main;\n"
 	  "gizmo foo { let a = 1; let b = 2; };\n"
 	  "export fn main() i32 = { return 0; };\n",
-	  0, 0 },
+	  0, 0, "expected top-level decl" },
 
 	/* control — a clean program builds + runs. */
 	{ "clean_ok",
@@ -111,11 +112,31 @@ run_build(const char *driver, const struct row *r, int i)
 }
 
 static int
-build_should_fail(const char *driver, const char *src, int i)
+filehas(const char *path, const char *needle)
 {
-	char s[64], tmpdir[64], cmd[1024];
+	char buf[8192];
+	FILE *f = fopen(path, "rb");
+	if (!f) return 0;
+	size_t n = fread(buf, 1, sizeof buf - 1, f);
+	fclose(f);
+	buf[n] = '\0';
+	return strstr(buf, needle) != NULL;
+}
+
+/* build_should_fail — the build must FAIL *and* emit `diag`. A crash
+ * (segfault) wraps as a nonzero "w6c failed" with no diagnostic, so the
+ * substring check distinguishes it from a clean reject (#20). cstage
+ * names the offending token ("expected top-level decl, got IDENT"),
+ * wwstage prints "parse: expected top-level decl"; the shared core is
+ * "expected top-level decl". */
+static int
+build_should_fail(const char *driver, const char *src, const char *diag,
+    int i)
+{
+	char s[64], tmpdir[64], cmd[1024], errf[64];
 	snprintf(s, sizeof s, "/tmp/undn_%d_%d.ww", getpid(), i);
 	snprintf(tmpdir, sizeof tmpdir, "/tmp/undn_%d_d_%d", getpid(), i);
+	snprintf(errf, sizeof errf, "/tmp/undn_%d_e_%d.txt", getpid(), i);
 
 	FILE *f = fopen(s, "wb");
 	if (!f) return -1;
@@ -123,11 +144,13 @@ build_should_fail(const char *driver, const char *src, int i)
 	fclose(f);
 
 	mkdir(tmpdir, 0755);
-	snprintf(cmd, sizeof cmd, "cd %s && %s build %s 2>/dev/null",
-	    tmpdir, driver, s);
+	snprintf(cmd, sizeof cmd, "cd %s && %s build %s >/dev/null 2>%s",
+	    tmpdir, driver, s, errf);
 	int rc = runwait(cmd);
+	int hasmsg = filehas(errf, diag);
 
 	unlink(s);
+	unlink(errf);
 	const char *base = strrchr(s, '/');
 	base = base ? base + 1 : s;
 	char outbin[128];
@@ -136,7 +159,8 @@ build_should_fail(const char *driver, const char *src, int i)
 	if (dot && strcmp(dot, ".ww") == 0) *dot = '\0';
 	unlink(outbin);
 	rmdir(tmpdir);
-	return rc == 0 ? -1 : 0;	/* build must NOT succeed */
+	/* build must NOT succeed AND must emit its diagnostic. */
+	return (rc != 0 && hasmsg) ? 0 : -1;
 }
 
 int
@@ -184,7 +208,7 @@ main(void)
 				}
 			} else {
 				if (build_should_fail(drivers[d].drv, rows[i].src,
-				    100 + i) != 0) {
+				    rows[i].diag, 100 + i) != 0) {
 					fprintf(stderr, "unknowndecl_reject[%s][%s]: "
 					    "built ok, expected a loud reject\n",
 					    drivers[d].name, rows[i].label);